1. Field of the Invention
This invention relates to a process for increasing the density of spray-dried, phosphate-reduced detergents.
2. Description of the Related Art
Spray-dried detergents of standard composition generally have apparent densities of 250 to 450 g/l (grams per liter) and, only in exceptional cases, of 480 g/l, depending on composition and procedure. In recent years, increasing interest has been shown in powders having higher apparent densities, for example in the range from 550 to 750 g/l, because they require less packaging material and, hence, provide for a saving of raw materials and a reduction in waste.
In addition, spray-dried detergents having apparent densities of from 550 to 900 g/l and processes for their production are known, for example in U.S. Pat. No. 4,552,681. However, the compositions in question here are special compositions rich in nonionic surfactants. An addition of anionic surfactants, particularly soaps, produces a marked reduction in apparent density to values below 500 g/l. The build-up granulation of individual detergent constituents in the presence of granulation liquids, such as water or alkali silicate solutions, also promotes high apparent densities. However, granulation with water generally requires the presence of relatively large amounts of salts which bind water of crystallization, mainly phosphates, such as tripolyphosphate, or soda. However, this also restricts freedom of formulation and complicates the production of P-free and P-reduced detergents. The spraying of nonionic detergents onto spray-dried or granulated powder also increases its apparent density, although the increase generally remains minimal. However, if relatively large amounts are used, the granulates are in danger of becoming tacky unless highly absorbent starting powders of special composition are used which again restricts freedom of formulation.
DE-A-25 48 639 describes a process for increasing the apparent density of granulated or spray-dried detergents in an apparatus which is known among experts as a "Marumerizer" and which is normally used to round off extruded particles of irregular shape. This apparatus consists of an upright cylinder having smooth side walls and a surface-roughened rotary plate which rotates in the lower part of the cylinder. It is primarily intended for intermittent operation. The largest available plants of this type, in which the rotary plate has a diameter of approximately 1 m, are only capable of accommodating a batch of at most 45 to 50 kg tower powder. For a residence time of approximately 10 minutes of the powder in the apparatus according to Example 3 of the cited DE-A, the throughput, based on an average hourly output of an average spray-drying tower of 5 to 25 t (tonnes), is far too low so that a very large number of "Marumerizers" constantly in operation would be necessary to keep pace with the tower output. On the other hand, it is uneconomical to operate the tower, including the expensive heating system, on only an intermittent basis and thus to adapt it to the low output of the granulator. Nor is it advisable to use the tower only sporadically for the production of the pregranulate, to store the pregranulate and to use the tower otherwise in the meantime. This is because, according to DE-A-25 48 639, the pregranulate or spray-dried powder must be subsequently processed as soon as possible, i.e. within a matter of minutes, in the "Marumerizer" to obtain a significant increase in powder density.
Accordingly, the object of the present invention is to avoid the disadvantages mentioned above and to provide a process which runs continuously, allows relatively high throughputs and relatively short residence times, guarantees maximal flexibility in regard to the quantity, physical condition and composition of the spray-dried powder and the production time and requires minimal capital investment and energy consumption.